169 
the climatic changes of the recurring seasons. This factor in 
the environment of the plankton is thus an ever changing one, 
but at the same time it runs an annual cycle of the same gen- 
eral character year after year with ever present minor varia- 
tions of a seasonal or local origin. The extremes of tempera- 
ture in bodies of water in this latitude are so divergent that 
they afford the basis for marked seasonal changes in both the 
constitution and the quantity of the plankton. Adaptations on 
the part of the organisms of the plankton to definite tempera- 
ture limits thus occur. 
Records of the temperature of the air and of the surface 
and the bottom water have been taken regularly at all stations 
where quantitative plankton collections were made. These 
are recorded in Table I. The temperatures were taken with a 
Negretti-Zambra self-recording thermometer from 1894 till May 
24,1898, after which time a Hick’s self-recording maximum-mini- 
mum thermometer was used. Under stable conditions no appre- 
clable variation was noted in the reading of the thermometers, 
but at the times of sudden change, as in the mingling waters of 
a rising flood, readings would sometimes vary as much as four 
or five degrees at one location and level. 
The temperature of the river water is influenced by a vari- 
ety of causes in addition to the immediate action of solar heat. 
The most prominent of these are the access of the tributary 
water from streams, springs, and impounding backwaters. The 
temperature of tributary streams, such as the Spoon River 
(Table LV.), is often, though not always, warmer in winter and 
colder in summer by several degrees than that of the main 
stream, as a result probably of the greater proportion of spring 
water and the greater nearness of the same to its subterranean 
source. A good illustration of this was to be seen along the 
eastern shore of Quiver Lake, where at low-water stages springs 
near the water’s margin kept up a continuous flow. The tem- 
perature of the water in summer was 54°, while in winter it 
fell only to 51°. The smaller tributary waters also respond 
more quickly to fluctuations of temperature than does the river 
